Valve for intravenous-line flow-control system

ABSTRACT

A valve ( 7 ) for use in controlling the flow of IV fluid from a source to a patient. A cassette may include along the fluid passage through the cassette, first and second membrane-based valves ( 6, 7 ) on either side of a pressure-conduction chamber ( 50 ), and a stopcock-type valve ( 20 ). The stopcock valve is preferably located downstream of the second membrane-based valve ( 7 ), which is preferably located downstream of the pressure-conduction chamber ( 50 ). The membrane defining the valving chamber of the second membrane-based valve ( 7 ) is preferably large and resilient, so that the valving chamber( 75 ) may provide a supply of pressurized intravenous fluid to the patient, when the valve ( 6 ) is closed and the stopcock valve ( 20 ) provides a restriction downstream of the valve ( 7 ). The pressure-conduction chamber ( 50 ) preferably has a membrane ( 41 ) that is stable in the empty-chamber position but relatively unstable in the filled-chamber position.

RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.08/917,537 filed Aug. 22, 1997, now U.S. Pat. No. 6,165,154 which, inturn, is a continuation-in-part of U.S. application Ser. No. 08/478,065filed Jun. 7, 1995, which issued as U.S. Pat. No. 5,755,683 on May 26,1998, which was concurrently filed with applications Ser. No.08/472,212, entitled “Intravenous-Line Flow-Control System” for aninvention by Heinzmann, Kamen, Lanigan, Larkins, Lund and Manning, whichissued on Jun. 30, 1998 as U.S. Pat. No. 5,722,637; Ser. No. 08/481,606,entitled “Intravenous-Line Air-Elimination System” for an invention byManning, Larkins, Houle, Kamen and Faust, which issued on Feb. 3, 1998as U.S. Pat. No. 5,713,865; and Ser. No. 08/477,380, entitled“Intravenous-Line Air-Detection System” for an invention by Larkins,Beavis and Kamen, which issued on Jun. 24, 1997 as U.S. Pat. No.5,641,892. All of these related applications are hereby incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to apparatus and methods for controllingflow through an intravenous line.

SUMMARY OF THE INVENTION

The invention is directed to a cassette for controlling the flow of IVfluid from a source to a patient. The cassette preferably includes,along the fluid passage through the cassette, first and secondmembrane-based valves on either side of a pressure-conduction chamber,and a stopcock-type valve. The stopcock valve is preferably locateddownstream of the second membrane-based valve, which is preferablylocated downstream of the pressure-conduction chamber.

In a preferred version of the cassette, which is primarily made out ofrigid material, the membrane for the second membrane-based valve isdisposed adjacent the housing, such that the rigid housing and themembrane define a valving chamber. One passage enters the valvingchamber at a first mouth located at the end of a protrusion of the rigidhousing into the valving chamber towards the membrane, and the valve mayprevent the flow of fluid therethrough when the membrane is forcedagainst the first mouth, by the control unit. The control valverestricts the flow of intravenous fluid from the valving chamber to thepatient, since it is located downstream of the valving chamber. Themembrane defining the valving chamber is preferably large and resilient,so that the valving chamber may provide a supply of pressurizedintravenous fluid to the patient, when the first mouth is sealed closedand when there is a restriction downstream of the valving chamber.

For the pressure-conduction chamber, a membrane is preferably disposedadjacent the rigid housing, so as to define a pressure-conductionchamber, wherein the rigid housing portion that defines thepressure-conduction chamber is generally dome-shaped. The membrane has afilled-chamber position, in which position the pressure-conductionchamber is substantially at its greatest volume, and an empty-chamberposition, in which position the pressure-conduction chamber is at itssmallest volume, and in which position the membrane rests against therigid housing and assumes the dome shape of the rigid housing. Themembrane preferably has a structure for creating instability in themembrane in the filled-chamber position. Preferably, this structure maybe actuated to create instability in the membrane in the empty-chamberposition. The rigid housing and the second membrane in the empty-chamberposition preferably define an unobstructed fluid passageway through thepressure-conduction chamber from the first to the secondpressure-conduction chamber mouth. Preferably, the structure forcreating instability in the membrane causes the membrane, when its atits full-chamber position, to collapse in the region of thepressure-conduction chamber's outlet mouth before collapsing nearer theinlet mouth. This structure helps force bubbles in the fluid upwardtoward the inlet mouth and the IV fluid source during a bubble-purgecycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of a cassette according to a preferredembodiment of the present invention.

FIGS. 2 and 3 show front and bottom views respectively of the cassetteof FIG. 1.

FIG. 4 shows a control unit for receiving and controlling a cassette,such as the cassette of FIGS. 1-3.

FIG. 5 shows a cross-section of the cassette of FIGS. 1-3.

FIG. 6 shows a rear view of the cassette and shows the fluid pathsthrough the cassette.

FIG. 7 shows a front view of the middle rigid panel of the cassette ofFIGS. 1-3.

FIGS. 8 and 9 show side and rear views respectively of the middle panelof FIG. 7.

FIG. 10 shows a partial cross-section of the middle panel of FIG. 7.

FIG. 11 is a cross-sectional detail of the control valve of the cassetteaccording to a preferred embodiment of the invention.

FIG. 12 shows a side view of an outer cylinder (a valve-seat member)having rigid and resilient elements that may be used in the controlvalve.

FIG. 13 shows a cross-sectional view of the cylinder of FIG. 12.

FIG. 14 depicts the relationship between the aperture of the FIG. 12cylinder and the groove used in the control valve.

FIG. 15 shows a cross-sectional view of the membrane that may be used inthe pressure-conduction chamber of the cassette shown in FIG. 1.

FIGS. 16 and 17 show front and rear views respectively of the FIG. 15membrane.

FIG. 18 shows a front view of the membrane used in the valve locateddownstream of the pressure-conduction chamber and upstream of thecontrol valve.

FIG. 19 shows a cross-section of the FIG. 18 membrane.

FIG. 20 is a schematic representing how the compliant membrane of FIG.18 may be used to regulate the pressure of fluid to the patient.

FIG. 21 is a graph depicting the advantage of using a compliant membranesuch as that shown in FIG. 18.

FIGS. 22 and 23 depict the preferred shape of the inlet valve to thepressure conduction chamber.

FIG. 24 shows a cross-sectional view of the inlet valve to the pressureconduction chamber.

FIG. 25 shows a preferred arrangement of teeth around the circumferenceof the control wheel.

FIG. 26 shows a front view of a cassette according to an alternativepreferred embodiment of the present invention.

FIG. 27 shows a front view of the membrane that may be used in thepressure-conduction chamber of the cassette shown in FIG. 26.

FIG. 28 shows a cross-sectional view of the membrane shown in FIG. 27along line B—B.

FIG. 29 shows a cross-sectional view of the membrane shown in FIG. 27along line 29;

FIG. 30 shows a cross-sectional view of the membrane shown in FIG. 27along line A—A.

FIG. 31 shows a perspective view of an alternative cassette which mayuse the membrane shown in FIGS. 27-30.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention includes a cassette for use in a system forcontrolling the flow of IV fluid to a patient, along the lines of thecassettes disclosed in U.S. Pat. Nos. 5,088,515 and 5,195,986. Apreferred embodiment of the cassette is depicted in FIGS. 1-3, whichrespectively depict top, front and bottom views of the cassette. Thecassette is used in a control unit, such as that described inabove-referenced U.S. Pat. No. 5,772,637, entitled “Intravenous-LineFlow-Control System,” which is similar to the control unit described inU.S. Pat. No. 5,088,515, which describe the use of pressure, preferablypneumatic pressure, for controlling the actuation of valves and theurging of fluid into and out of a pressure-conduction chamber. Inaddition to performing the function of a pump urging fluid through theIV line, the pressure-conduction chamber can measure the amount of IVfluid being delivered to the patient as well as detect the presence ofbubbles in the IV fluid in the pressure-conduction chamber. Preferredmethods of detecting and eliminating air bubbles from the IV fluid arediscussed in the above-referenced patent applications for“Intravenous-Line Air-Detection System” and “Intravenous-LineAir-Elimination System,” now U.S. Pat. Nos. 5,641,982 and 5,713,865,respectively. FIG. 4 depicts a preferred version of a control unit 10.Control unit 10, which has a user-interface panel 103 containing a keypad and a display so that the status of the IV fluid delivery may bemonitored and modified by medical personnel. The cassette is slippedbehind door 102, and by turning handle 101 the door is pressed againstthe cassette, which in turn is then pressed against the main housing ofthe control unit 10. The main housing 104 preferably includes mechanicalmeans for actuating membrane-covered valves and for applying a pressureagainst the membrane of the pressure-conduction chamber. The mainhousing 104 also includes means for turning the control wheel of thecassette.

Referring to FIG. 2, the main components of the preferred embodiment ofthe cassette are a first membrane-based valve 6, a pressure-conductionchamber 50, a second membrane based valve 7 and a stopcock-type controlvalve 20. Valve 6 controls the flow to the pressure-conduction chamber50 from the inlet 31 to the cassette, which is connected to an IV line,which in turn is connected to a source of IV fluid. The secondmembrane-based valve 7 and the control valve 20 together are used tocontrol the flow of fluid from the pressure-conduction chamber 50 to theoutlet to the cassette 33, which is connected to the IV line leading tothe patient.

The rigid housing 15 of the cassette is made primarily from three rigidpanels. A front panel 17, a middle panel 18, and a rear panel 16, allthree of which can be seen in FIGS. 1 and 3. The front panel ispreferably molded integrally with the outer collar 21 of the controlvalve 2. The wheel 20 of the control valve 2 preferably includes ribs281 and/or teeth mounted along the circumference 29 of the knob 20.(FIG. 25 shows a preferred arrangement of teeth around the circumference29 of the control knob 20.) The teeth and/or ribs 281 may be engaged bythe main housing 104 of the control unit 10, so that the control unit 10may change the resistance that the control valve 2 exerts on the IVfluid passing through the valve.

The cassette may also be used without the control unit 10. In that case,the control wheel 20 may be turned by hand. When disengaged from thecontrol unit 10, the membrane of the pressure-conduction chamber 50 ispreferably collapsed so that it rests against the rigid rear wall 59 ofthe pressure-conduction chamber 50. With the membrane in this collapsedstate, IV fluid may still easily flow through the pressure-conductionchamber 50 through a raised portion 35 of the rear wall 59. This raisedportion 35 defines a conduit 36 leading from the inlet mouth of thepressure-conduction chamber 50 to the outlet mouth of thepressure-conduction chamber, as can be seen in FIG. 6. FIG. 6 shows thefluid paths leading through the cassette. As noted above, fluid entersthe cassette through the inlet 31, whence it flows through a fluid pathto valve 6. The fluid then enters the valving chamber of valve 6 throughan inlet port 62. An outlet port 61 is preferably mounted on aprotrusion so that pressure from the pressure-conduction chamber 50 isless likely to force the membrane to lift from the outlet port 61. Fromvalve 6 the fluid passes to the inlet mouth 56 of thepressure-conduction chamber 50. The pressure-conduction chamber is seenin the cross-sectional view of FIG. 5. A membrane 41 allows pressurefrom the control unit 10 to be applied to the fluid in thepressure-conduction chamber 50 without the fluid coming into contactwith the control unit 10. When the membrane 41 is in its collapsedposition resting against rigid wall 59, as shown in FIG. 5, fluid canstill pass from inlet valve 56 through conduit 36 to the outlet valve57. After passing through the pressure-conduction chamber 50, the fluidflows to the second membrane-based valve 7, which included an inletmouth 73, which is mounted on a protrusion 72 in similar fashion to theoutlet port 61 of the first membrane-based valve 6. The secondmembrane-based valve's inlet mouth 73 and the protrusion 72 on which itis mounted can be seen in the cross-sectional view of FIG. 5. Like theoutlet port 61 of the first membrane-based valve, the inlet mouth 73 maybe closed by the application of pressure by the control unit on amembrane; a first portion 71 of the membrane that closes off the inletmouth 73 can be seen in FIG. 5. After passing through the outlet mouth76 of the second membrane-based valve 7, the fluid passes to the inlet77 of the stopcock-type control valve, which inlet can be seen in bothFIGS. 5 and 6. After passing through the control valve and the fluidpath 78 exiting from the control valve, the fluid passes to the outletof the cassette 33 and to the IV line leading to the patient.

FIG. 7 shows a front view of the rigid middle portion of the cassette,and FIG. 8 shows a side view of the middle rigid panel 18. The middlerigid panel 18 defines the cassette inlet 31 and outlet 33, acircumferential portion of the pressure-conduction chamber 50, and port62, outlet port 61, inlet mouth 73, and outlet mouth 76 of the twomembrane-based valves 6 and 7. The protrusions 63 and 72 of the outletport 61 and inlet mouth 73 can also be seen in FIG. 7. FIG. 9 shows arear view of the middle rigid panel 18 shown in FIGS. 7 and 8. Theports/mouths 61, 62, 73, 76 can also be seen in FIG. 9. FIG. 10 shows apartial cross-section of the middle rigid portion. The cross-sectionshows the outer collar 21 of the control valve, which is integrallymolded with the rest of the middle rigid portion. The outer collar 21defines a hollow area 22 and a fluid path 23 leading from the hollowarea 22.

FIG. 11 shows a cross-section of an assembled control valve 2 that maybe used in a cassette according to the present invention. Just inside ofthe outer collar 21 is a valve-seat member 22 fixedly attached to theouter collar 21 so that the valve-seat member 21 does not rotate withrespect to the rest of the cassette. The valve-seat member 21 isdepicted in greater detail in FIG. 12 and in cross-section in FIG. 13.The valve-seat member 22 also defines a hollow area, which accepts theshaft 220 of the control wheel 20, so that the control wheel's shaft 220rotates with the control wheel 20. The valve-seat member 22 is comprisedmostly of rigid material, but importantly it also includes molded-overresilient material, which is used to form sealing O-rings. Thisresilient material forms an O-ring 26 around the base of the valve-seatmember 22; the rigid portion of the base defines a passage 222,connecting the valve inlet 77 to passage 24. The resilient material 25also provides a seal around an aperture 251 in the circumferentialsurface of the member 22. At the end of the member 22 opposite the inletpassage 222 is an inner O-ring 27 which forms the seal between thecontrol wheel's shaft 220 and the valve-seat member 22. The O-ring 26around the exterior circumference of the base provides a seal betweenthe outer circumferential wall of the valve-seat member 22 and the innercircumferential wall of the outer collar 21. Likewise, the O-ring 25around the circumferential port 251 may provide a seal between the outercircumferential wall of the valve-seat member 22 and the innercircumferential wall of the outer collar 21. Together, O-rings 25, 26prevent fluid from leaking between the valve-seat member 22 and theouter collar 21. Importantly, the O-ring 25 of port 251 also provides aseal between the valve-seat member 22 and the shaft 220, so that whenthe valve is in the fully closed position no flow is permitted betweenpassageway 24 of shaft 220 and the port 251 of the valve-seat member 22.

The advantage of this design over previous stopcock valves is that theouter diameter of the shaft 220 may be slightly less than the innerdiameter of the valve-seat member 22, whereas previous stopcock valvesrequired an interference fit between the inner and outer components. Itwill be appreciated that the stopcock valve of the present invention mayuse frusto-conical-shaped members instead of cylindrical members. Theinterference fit of prior-art devices created a great deal of resistancewhen the stopcock valves were turned. The use of O-rings in the stopcockvalve of the present invention avoids the need for this interference fitand the greater torque required for turning the valve resulting from theinterference fit. O-ring 27 prevents leaking from the space between thevalve-seat member 22 and the shaft of the control wheel 20.

The valve-seat member is preferably made in a two-part molding process,wherein the rigid portion is first molded and then the softer resilientmaterial is over-molded onto the rigid portion. Channels may be providedin the initially molded rigid portion so that the resilient material mayflow to all the desired locations; this results in columns of resilientmaterial 28 connecting the areas of resilient material through thesechannels. The valve-seat member 22 is preferably molded separately fromthe rest of the cassette, and when the cassette is assembled thevalve-seat member 22 is placed in the hollow defined by the outer collar21 of the middle panel 18, and aligned so that aperture 251 lines upwith passageway 23. (The shape of the outer diameter of the valve-seatmember 22 and the inner diameter of the outer collar 21 may becomplementarily shaped so that the valve-seat member must align properlywith the aperture 251 and the passageway 23 lined up.) Then, the frontrigid panel 17 is ultrasonically welded (along with the rear rigid panel16) to the middle rigid panel 18, and the valve-seat member 22 is thenheld in place in the hollow area defined by the outer collar 21. Theouter circumference of the valve-seat member 22 may be a bit smallerthan the inner diameter of the outer collar 21; O-rings 25, 26 preventfluid from flowing from the passages 77 or 23 to point 19. This designof the valve-seat member 22 avoids the need for tight tolerances in thevarious components of the valve 2. The control wheel's shaft 220 may beinserted into the hollow area defined by valve-seat member 22 after therest of the valve has been assembled. The shaft 22(0 is held in place bya lip 161around the inner circumference of the hollow area defined bythe rear rigid panel 16

When the valve 2 is fully opened, the circumferential aperture 251 islined up with the fluid passage 24 in the shaft 220. When the valve isfully closed there is no fluid communication between the aperture 251and the fluid passage 24. The outer circumferential surface of the shaft220 preferably includes a groove extending circumferentially around theshaft's outer circumferential wall from the terminus of the fluidpassage 24 at the outer circumferential wall; the groove tapers incross-sectional area and does not extend all the way around the outercircumference of the shaft 220. The groove provides greater control ofthe flow rate. FIG. 14 shows the respective locations of the groove 231,which is located on the outer circumference of the shaft 220 and thecircumferential aperture 251 of the valve seat member 22. As theaperture 251 rotates to the right, in the FIG. 14 perspective, theresistance to flow increases, until the groove 231 tends and theaperture 251 loses fluid communication with the groove 231, at whichpoint flow is completely shut off through the control valve 2. As theaperture 251 rotates to the left, in the FIG. 14 perspective, theresistance to flow decreases. Preferably, the groove 231 is longer thanthe diameter of the aperture 251, so that the flow rate may becontrolled more finely.

As noted above, the cassette may be used independently of the controlunit 10. When the cassette is used in this manner it is preferable thatthe membrane 41 rest against the rigid back 59 of thepressure-conduction chamber 50 so as to minimize the volume of theconduit 36 for fluid passing through the pressure conduction chamber 50.If the membrane 41 were too flexible and the volume of thepressure-conduction chamber 50 varied widely, medical personnel would beunable to rely on a quick visual inspection of the rate of dripping inthe drip chamber to indicate a steady, desired flow rate through the IVline. Thus, it is desired that the structure of the membrane 41 be suchthat it tends to rest against wall 59 unless and until a sufficientpressure differential is created across the diaphragm 41. This pressuredifferential is preferably caused by a negative gas pressure caused bythe control unit 10. Although it is desired to manufacture the diaphragm41 so that it has some tendency to rest against wall 59, it is desiredto make the diaphragm 41 floppy in the other direction so that lesspressure is required to move it from its position when thepressure-conduction chamber 50 is full, the “filled-chamber” position.It is also desired that the measurement gas provided by the control unit10 against the outer face of the membrane 41 be at substantially thesame pressure as the fluid on the inner side of the membrane 41 in thepressure-conduction chamber 50.

By molding the diaphragm 41 in the shape of a dome corresponding to thatof the rigid wall 59, the diaphragm will have a tendency to remain inits position, as shown in FIG. 5, resting against wall 59 when thechamber 50 is at its lowest volume, the “empty-chamber” position.However, when the diaphragm 41 is molded in this way, it also tends toremain in the filled-chamber position, in other words, when thediaphragm 41 is bulging convexly outward from the cassette. This convex,filled-chamber position can be made unstable by adding additionalmaterial on the outer, usually concave surface of the diaphragm 41. Thisadditional material 43 can be seen in the cross-section of a preferredembodiment of the diaphragm as shown in FIG. 15. The diaphragm 41 shownin FIG. 15 is molded in the position shown and has a tendency to remainin that position. When the chamber is filled with fluid, the normallyconcave side of the diaphragm becomes convex, and the additionalmaterial 43 is subject to an additional amount of strain since it is atthe outer radius of this convex, filled-chamber position. The diaphragm41 shown in FIG. I5 also includes an integrally molded O-ring 44 aroundits circumference for mounting and sealing the diaphragm 41 in thecassette. FIG. 16 shows a view of the exterior side of the diaphragm 41of FIG. 15. This surface of the diaphragm 41 is normally concave whenthe diaphragm is in the empty-chamber position. The additional material43 can be seen in the view of FIG. 16. FIG. 17 shows the interior sideof the diaphragm 41 of FIG. 15. This side is normally convex when thediaphragm 41 is in the empty-chamber position. Thus, as a result ofmolding the diaphragm so that its inner surface has a smooth constantradius and the outer surface has additional material, which therebyinterrupts the smoothness and constant radius of the rest of the outerface of the diaphragm, the diaphragm 41 has the desired tendency toremain in the empty-chamber position while being unstable in thefilled-chamber position.

By positioning this additional material 43 near the outlet mouth 57 ofthe pressure-conduction chamber 50, the collapse of the diaphragm 41from its filled-chamber can be somewhat controlled so that the diaphragmtends to collapse first in the lower portion of the pressure-conductionchamber near the outer mouth 57 before further collapsing in the upperregion of the pressure conduction chamber nearer the inlet mouth 56. Thecassette is preferably mounted in the control unit with a slight tilt sothat the passage 36 is vertical and the inlet mouth 56 is at the verytop of the chamber 50 and the outlet mouth 57 is at the very bottom ofthe chamber 50. This orientation permits the bubbles that may be presentin the chamber 50 to gravitate towards the inlet mouth 56, which is atthe top of the chamber. In a preferred method of eliminating the bubblesfrom the IV fluid, as described in the above-referenced, concurrentlyfiled application for “Intravenous-Line Air-Elimination System,” anybubbles that are detected by the control unit in the pressure conductionchamber 50 are forced by pressure from the control unit against theexternal surface of the membrane 41 up to the inlet mouth 56 to thecassette inlet 31 up the IV line to the fluid source, sometimes afterseveral purging and filling cycles. When purging the bubbles from thechamber 50 through the inlet mouth 56 it is preferred that the chambercollapse at its bottom first so that the membrane does not interferewith bubbles moving upwards through the chamber 50.

Thus, the additional material 43 creates an instability in the membrane41 when the membrane is in the filled-chamber position, thereby makingthe membrane more likely to collapse from the filled-chamber positionthan a membrane that did not have the additional material. Theadditional material 43, however, does not create an instability in themembrane 41 when the membrane is in the empty-chamber position. In manysituations it is desirable to be able to introduce some instability intothe membrane when the membrane is in the empty-chamber position. Byintroducing such instability into the membrane, less negative pressureis needed to move the membrane from its empty-chamber position.

To create an instability in the empty-chamber position, apressure-relief tab 143 may be added to the membrane 141 as shown inFIG. 26. The pressure-relief tab 143 extends from the exterior surface145 near the edge of the membrane 141, as can be seen in thecross-sectional view of FIG. 29. FIG. 28 shows another cross-sectionalview, which view does not pass through the pressure-relief tab 143, andFIG. 27 shows a front view of the membrane 141. The tab 143 may beactuated by an actuator 149 (shown in FIG. 30) mounted in the controlunit. When it is desired to introduce instability into themembrane—which will typically be whenever it is desired to fill apreviously empty chamber 50—the actuator 143 forces the tab 143 towardsthe O-ring 144. This action pulls the portion of the membrane 141 nearthe tab 143 away from the cassette's rigid wall, which partially definesthe pressure-conduction chamber 50. The tab 143 is located near theinlet mouth of the chamber 50 so that, when the actuator 149 pulls aportion of the membrane 141 away from the rigid wall, a pocket of spaceis formed into which the fluid can flow. By supplying a negativepressure to the exterior surface 145 of the membrane 141, the controlunit may cause more liquid to be drawn into the pressure-conductionchamber 50. Less negative pressure is needed to move the membrane 141out of the empty-chamber position, when the actuator 149 has urged thetab 143 towards the O-ring 144 and the rigid portion 117 of the cassetteadjacent the tab 143.

If it is desired to make the membrane 141 stable in the empty-chamberposition, the control unit may cause the actuator 149 to be returned tothe non-actuating position, so that the tab 143 may return to its normalposition, extending outwardly from the cassette. As noted above, whenthe membrane is in the empty-chamber position, IV fluid may flow throughthe pressure-conduction chamber 50 through a conduit defined by raisedportion of the rear wall (see FIGS. 3, 5 and 6) and leading from theinlet mouth of the pressure-conduction chamber 50 to the outlet mouth ofthe pressure-conduction chamber.

The pressure-reduction tab 143 also creates an instability in thefilled-chamber position. When the pressure-conduction chamber 50 isfilled with liquid, the exterior surface 145 of the membrane 141 becomesconvex, rotating the tab 143 towards the O-ring 144, so that the tab 143is urged against the rigid portion 117 of the cassette. In thisposition, the tab 143 creates pressure on a portion of the membrane 141so as to make the membrane less stable in the filled-chamber position sothat the control unit needs to create less positive pressure to collapsethe membrane 141 from its filled-chamber position.

FIG. 31 shows a cassette 215 that may be used in a bed-side pharmacysystem, such as that described in the concurrently filed patentapplication for “System, Method and Cassette for Mixing and DeliveringIntravenous Drugs” bearing assigned Ser. No. 08/916,890, and which listsKamen, Grinnell, Mandro, Gilbreath, Grant, Demers, Larkins and Manningas inventors, now abandoned in favor of continuation-in-partapplication, assigned Ser. No. 09/137,025 which application isincorporated herein by reference. Such a cassette may also use amembrane 241 having a pressure-reduction tab 243, which creates someinstability in the filled-chamber position and which may be actuated tocreate some instability in the empty-chamber position.

Returning to the cassette 15 shown in FIGS. 1-3, a preferred membranedesign for the second membrane-based valve 7 is shown in FIGS. 18 and19. This membrane has an O-ring 78 for mounting and sealing the inlet;membrane onto the cassette (like the lip 44 on the membrane 41 for thepressure-conduction chamber, and like the circular membrane, which isnot shown, for the first membrane-based valve 6). This membrane has afirst portion 71, which is used to seal off the inlet mouth 73 locatedon protrusion 72 (see FIG. 5). The control unit 10 exerts a pressureagainst this portion of the membrane 71 mechanically, in order to closeoff the valve 7. A second compliant portion 74 of the membrane issufficiently compliant so that when the control valve 2 is sufficientlyrestricting flow out of the outlet 76 of the second membrane-based valve7, the compliant portion 74 of the membrane will expand outwardly so asto hold, under pressure, a volume of IV fluid. This design is desirableso that when the inlet mouth 73 is closed, because thepressure-conduction chamber needs to be refilled, the fluid stored inthe valving chamber (item 75 in FIG. 5) is available to be dispensedthrough the control valve 2.

FIG. 20 shows a schematic for an electrical model of the operation ofthe second membrane-based valve 7 working in conjunction with thestopcock-type control valve 20. When the valve leading from the outlet57 of the pressure-conduction chamber 50 is open, permitting flow fromthe pressure-conduction chamber through valve 7, and if the stopcockvalve 20 is set to provide a large amount of resistance to the flow fromvalve 7 to the patient, the valving chamber 75 and its correspondingcompliant membrane portion 74 can accumulate a “charge” of fluid, muchlike a capacitor, as shown in FIG. 20. When first portion 71 is thenurged against inlet mouth 73 closing off flow from thepressure-conduction chamber 50, the charge of fluid in the valvingchamber 75 is urged by the compliant membrane portion 74 to continueflow through the stopcock valve 20. As fluid exits the valving chamber75, the pressure of the fluid decreases as the compliant portion 74 ofthe membrane returns to its unstretched state. FIG. 21 shows a graphdepicting the pressure of the IV fluid being delivered to a patient overtime as outlet valve 71, 73 is closed at time t₁ and reopened at t₂. Asolid line depicts the pressure to the patient without a compliantmembrane portion 74 design. With a compliant membrane portion 74, thesharp drop off in pressure at t₁ is eliminated or ameliorated. If thestopcock valve is nearly closed so that only a small trickle of fluid isallowed to flow through it, the design of the compliant membrane portion74 will greatly smooth out the delivery of fluid, as long as the timebetween t₁ and t₂ is not too long. When the stopcock valve 2 is fullyopen a sharp drop in pressure may still be expected at time t₁.

As noted above (and as described in the above-referenced U.S. Pat. No.5,713,865, entitled “Intravenous-Line Air-Elimination System”), when anair bubble is being purged from the pressure-conduction chamber 50, itis preferably forced up through the chamber's inlet valve 56 (which inthis air-elimination mode is acting as an outlet). Preferably, the inletport 56 is shaped so that a small bubble will not tend to stick to anedge of the port while allowing liquid to flow past it. To prevent suchsticking of a small bubble, the port 56 preferably flares out so thatthe corner where the port 56 meets the inner wall of thepressure-conduction chamber 50 is greater than 90°, making the cornerless likely a place where the bubble will stick. However, the mouth ofthe port 56 cannot =be so large that liquid can easily flow by thebubble when fluid is exiting the pressure-conduction through the port56. In order to accomplish this, the port must be sized and shaped sothat the surface tension of the IV fluid being forced upward from thepressure-conduction chamber 50 forces a bubble located at the port 56 upthrough the inlet valve 6. It is also preferable that the port 56 besized and shaped so that when liquid is pulled back into thepressure-conduction chamber 50, the bubble can hover near the port asliquid passes around it. A preferred inlet port 56 shape is shown inFIGS. 22 and 23. The port's size increases from the end 57 that connectsto the IV line's upper portion to the end 58 leading into thepressure-conduction chamber. FIG. 24 shows a cross-section of the inletvalve 56. It has been found that providing an inlet port to thepressure-conduction chamber with this shape improves the air-eliminationsystem's ability to purge bubbles from the chamber. Using a port such asthat shown in FIGS. 22-24 in conjunction with the membrane 41 of FIGS.15-17 helps force bubbles more quickly out of the pressure-conductionchamber when attempting to purge the bubbles back through the cassette'sinlet 31 to the IV source.

FIG. 25 shows a preferred arrangement of teeth around the circumference29 of the control wheel 20. The teeth provide means for a gear in thecontrol unit 10 to engage securely the control wheel's circumference-inparticular, a gear that is used to prevent the free flow of fluidthrough the cassette when the cassette is removed from the control unit10. When the door 102 of the control unit 10 is being opened, the gearturns the control wheel 20 to close the stopcock-type valve 2, therebystopping all flow through the cassette and preventing free flow. Toensure that the gear does not continue turning the wheel 20 once thevalve 2 has been closed off entirely, a sector 92 along the wheel'scircumference is left free of teeth. When the wheel 20 is turned enoughso that the gear is adjacent this toothless sector 92, the valve 2 isfully closed. The lack of teeth prevents the gear from continuing toturn the wheel; thus, the wheel cannot be turned too much.

Although the invention has been described with reference to severalpreferred embodiments, it will be understood by one of ordinary skill inthe art that various modifications can be made without departing fromthe spirit and the scope of the invention, as set forth in the claimshereinbelow.

What is claimed is:
 1. A valve for smoothing the delivery of intravenousfluid from a source to a patient comprising: an occludable inlet mouth;an expandable valving chamber disposed downstream from and in fluidcommunication with the inlet mouth; and a restrictive outlet mouthdisposed downstream from and in fluid communication with the expandablevalving chamber, wherein the outlet mouth is capable of variablyrestricting delivery to the patient by providing alternatively (i) afully open position, (ii) a partially open position and (iii) a closedposition; so that the chamber is expandable to accept and hold underpressure a charge of fluid via the inlet mouth when the outlet mouthsufficiently restricts delivery, and so that the charge is capable ofdelivery to the patient when the inlet mouth is occluded, the chamberapplying pressure on the charge while returning to an unstretched statesuch that the charge is urged to continue flowing through the outletmouth.
 2. A valve according to claim 1 further comprising: a compliantmembrane, the membrane defining the expandable valving chamber.
 3. Avalve according to claim 2, wherein a portion of the compliant membraneis capable of occluding the inlet mouth.
 4. A membrane-based valve forsmoothing the delivery of intravenous fluid from a source to a patientcomprising: a rigid housing, the housing providing a first passage, afirst mouth, a second passage, a second mouth; and a compliant membrane;the housing and the compliant membrane coupled, defining a valvingchamber; the first passage entering the valving chamber at the firstmouth such that flow of fluid via the first passage into the chamber iscapable of being prevented when the compliant membrane is forced againstthe first mouth, the second passage exiting the valving chamber at thesecond mouth so that a charge of fluid is capable of being urged by thecompliant membrane to continue flow from the valving chamber into thesecond passage via the second mouth and may be provided to the patientwhen the compliant membrane is forced against the first mouth.
 5. Avalve according to claim 4, wherein the first mouth is located at an endof a protrusion provided by the rigid housing, the protrusion extendinginto the valving chamber towards the compliant membrane.
 6. A valvecomprising: an occludable inlet mouth; a valving chamber disposeddownstream from and in fluid communication with the inlet mouth, thevalving chamber capable of expanding so as to accept and retain a chargeof fluid under pressure; and an outlet mouth disposed downstream fromand in fluid communication with the valving chamber, so that the chargeof fluid is urged, while the valving chamber is returning to itsunstretched state, to flow therethrough when the inlet mouth isoccluded, the outlet mouth capable of variably restricting delivery tothe patient by providing alternatively (i) a fully open position, (ii) apartially open position and (iii) a closed position.
 7. A valveaccording to claim 6, wherein the valving chamber is defined by amembrane.
 8. A valve according to claim 7, wherein a portion of themembrane is capable of occluding the inlet mouth.